(a) Field of the Invention
The present invention relates to a device and method for discharging a capacitor of an input filter of a power supply. In particular, the present invention relates to a power supply for improving power efficiency and reducing standby power.
(b) Description of the Related Art
Power supplies are widely used in order to provide stable power to computers, home appliances, communication devices, etc. As issues such as environmental pollution and energy resource exhaustion have been recently raised, the efficiency of power supplies grows more important. In particular, standby power of the power supply has been focused on.
FIG. 1 shows a circuit diagram of a conventional power supply.
As shown in FIG. 1, the conventional power supply is provided with an AC power source Vac to generate a regulated DC voltage Vdc, and includes an input filter 10, a bridge diode BD, a capacitor Cin, and a DC-DC converter 20.
The input filter 10 stabilizes the AC power source Vac and reduces electromagnetic interference (EMI), and includes a first input capacitor C1, a discharging resistor Rd, a choke coil L1, and a second input capacitor C2. Two terminals of the first input capacitor C1 are connected to two terminals of the AC power source Vac, respectively. Two terminals of the discharging resistor Rd are also connected to two terminals of the AC power source Vac, respectively. Two input terminals of the choke coil L1 are connected to two terminals of the AC power source Vac, respectively. Two output terminals of the choke coil L1 are connected to two terminals of the second capacitor C2, respectively.
The bridge diode BD is composed of four diodes, and includes two input terminals and two output terminals. The two input terminals of the bridge diode BD are connected to two output terminals of the choke coil L1, respectively. The two output terminals of the bridge diode BD are connected to two terminals of the capacitor Cin, respectively. The bridge diode BD rectifies an EMI-reduced AC power source. The capacitor C1 smooth a rectified AC power source to generated an input voltage Vin of the DC-DC converter 20.
The DC-DC converter 20 converts the input voltage Vin into a regulated DC voltage Vdc.
Because the first input capacitor C1 and the second input capacitor C2 are charged by a high voltage, discharge of the first input capacitor C1 and the second input capacitor C2 is required when supply of the AC power source Vac stops.
For this purpose, the prior power supply has the discharging resistor Rd to discharge the first input capacitor C1 and the second input capacitor C2. However, since the discharging resistor Rd continues to be provided with the AC power source Vac, it reduces efficiency of the power supply. Also, in case of no load, the discharging resistor Rd causes large standby power.
Moreover, as the size of the first input capacitor C1 is larger, the size of the second input capacitor C2 is larger, or the discharge speed is more rapid, the size of the discharging resistor Rd is required to be small, so it causes lower efficiency of the power supply.